If you're willing to get your hands dirty and learn a bit of *nix I recommend pf on OpenBSD which is _very_ flexible and will let you 'tarpit' spammers (with spamd) if you wish. It's free and it'll run very well on a pII 350mhz with 128m of RAM. It is a bit of a learning curve if you're a Windows only guy but well worth it IMHO.
Even easier is IMGate/postfix's "anvil" feature which will dynamically smtp-blocks/rate-limits any IP that connects to postfix more than x times in y minutes.
anvilled IPs connect to port 25, postfix sends an immediate SMTP 421 code, and hangs up. postfix can probably do that 200 times/second without impacting legit operation.
I would say the majority of msgs to unknown users come from subscriber access networks of millions infected PCs, each of which doesn't attack any one MX at a high rate of attempts, so rate limiting is not helpful.
Len
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